The present invention relates in general to communication systems, and is particularly directed to a hybrid antenna reflector that contains an interior solid reflector region, adjacent at its perimeter to a ring-shaped dichroic reflector region. The solid interior region is reflective to RF energy at each of first and second spaced apart frequency bands, while the dichroic reflector region is reflective at the first frequency band, but nonreflective at the second frequency band. This allows the hybrid reflector antenna to realize the same beamwidth coverage at each of first and second spaced apart frequency bands.
Spaceborne reflector antenna systems that have been deployed or proposed to date for multiple spot (terrestrial) coverage illumination at widely separated spectral regions of an elevated frequency band (such as Ka-Band as a non-limiting example) have required separate and differently sized reflector structures for their transmitter (T) and receiver (R) subsystems, in order to achieve the same (T/R) beamwidth coverage per spot. If a geostationary satellite based antenna system is intended to provide simultaneous coverage of a plurality of adjacent terrestrial regions, such as the oval regions diagrammatically shown in the beam pattern coverage map of the United States of FIG. 1, the satellite, such as that shown at 10 in FIG. 2, must be configured to support a limited number of reflector antenna pairs (e.g., four pairs A, B, C, D, or eight individual reflector antennas), each transmit-receiver reflector antenna pair comprising two differently sized antenna reflectors and attendant feed subsystems operating at respectively spaced apart frequency bands.
To provide for spot coverage, such as the example shown in FIG. 1, a number of transmit and receive reflector pairs is required. Furthermore, for accurate spot pointing, it may be required that each reflector be mounted to its own dedicated pointing subsystem. Not only does this add considerable mass and volume to an already physically cumbersome hardware and RF interface problem, particularly where the mounting real estate and payload parameters of spaceborne components are inherently restricted, but substantially increases cost of design and space-deployment.
In accordance with the present invention, these shortcomings of conventional spaceborne reflector antennas are effectively obviated by a hybrid antenna reflector architecture that is configured to provide the same beamwidth (projected terrestrial spot) coverage at widely spaced apart frequency bands, so that only one reflector is required to illuminate the same sized spot on the earth for an antenna simultaneously operating at widely spaced apart frequency bands. As will be described, the hybrid antenna reflector of the invention contains a generally circular or polygonal, interior solid parabolic or alternately shaped reflector sector or region, that is adjacent at its perimeter to a generally ring-shaped or annular dichroic reflector sector. Each sector may be constructed of assembled panels using low coefficient of thermal expansion (CTE) composite laminates for structural integrity and for reduced thermal distortion of the reflector surfaces. The solid interior sector is reflective to RF energy at each of a pair of relatively widely spaced apart frequency bands, such as, as a non-limiting example, spectrally separate transmit and receive portions of a given operating band or bands, while the exterior dichroic reflector sector is reflective at a first (e.g., lower) frequency band, but is non-reflective (e.g. transmits or absorbs) at a second (e.g., higher) frequency band. The interior and exterior sectors are aligned such that a continuous RF reflective surface is formed for the first (lower) frequency band.
The inner radial dimension of the exterior dichroic reflector sector is defined so that the effective aperture or beamwidth of the hybrid antenna reflector is the same for each of the two spaced apart bands at which the antenna is intended to operate. This allows a single hybrid antenna reflector to produce one or multiple beam pattern(s) that cover(s) the same illuminated terrestrial region(s), and thereby reduces by a factor of two the number of antennas (reflectors and feeds) that would otherwise have to be mounted on a satellite to obtain simultaneous coverage of a single terrestrial region or a plurality of terrestrial regions.
For structural integrity to the satellite bus, the rear surface of the hybrid antenna reflector architecture of the invention is mounted to a stable backing support structure, such as a generally regular polygon-shaped frame formed of interconnected struts made of a material whose coefficient of thermal expansion is relatively low and compatible with that of the hybrid antenna reflector. The backing frame is integrally joined with the satellite via an actuator coupling joint, which, when combined with an actuator mechanism system, enables deployment and/or proper pointing of the reflector system. The actuator coupling joint may be radially displaced from the exterior perimeter of the exterior dichroic sector, so that it may be readily affixed to an actuator installed on the satellite.
Because it is adjacent to the rear side of the antenna""s exterior dichroic sector, the backing frame is a potential reflector of RF energy passing through the exterior dichroic sector. To prevent unwanted reflections by the backing structure, the portion of the backing support frame behind the exterior dichroic sector may be configured to deflect, absorb, transmit, or otherwise minimize reflection of RF energy that has passed through the exterior dichroic sector towards the coverage region.